Listening with just one ear seriously degrades signal segregation and communication. Many patients function with noticeable asymmetry in hearing. We propose to study the effects of asymmetric hearing in three patient populations. In each group, individuals have one deaf ear; however, they differ in their better hearing ear, which has either normal hearing, moderate hearing loss, or hears with a cochlear implant. In Aim 1, we test the hypothesis that auditory processing in the intact (normal) ear is disrupted in adults with unilateral hearing loss. Specifically, we predict decreased processing of spectrally and temporally modulated sounds in individuals with unilateral deafness compared to normal hearing controls listening with one ear. In Aim 2, using imaging methods, we test the hypothesis that unilateral deafness leads to reorganization in the normal cortical asymmetry in processing spectral and temporal sounds. We will also examine the consequences of unilateral deafness on correlated activity between selected auditory regions in the deprived auditory cortex and the rest of the brain. In Aim 3a, we hypothesize that residual hearing in one ear reduces the negative effect of deafness in the opposite, untreated ear. Adult patients have one ear that meets current cochlear implant candidacy and one ear with moderate hearing loss. The poorer ear is implanted. Results are compared to individuals with deafness in both ears who receive one implant. In Aim 3b, we hypothesize that electric hearing through a cochlear implant in one ear reduces the negative effect of deafness in the opposite, untreated ear. For both children and adults who receive sequential cochlear implants (one ear implanted months to years after the first), we determine the relationship between sequentially implanted ears using roving-source speech recognition, multi-source simulation of real-world noise environments, and investigation of hearing handicap. The information obtained from these studies will provide an urgently needed basis for expanding criteria for implantation in one ear and establishing new criteria for implantation in both ears for children and adults. In addition, the proposed translational research will advance understanding of plasticity and dominance of the neural pathways in deafness and guide intervention for patients with asymmetric hearing, either due to acoustic asymmetry or unilateral cochlear implantation. Listening with just one ear results in poor speech understanding in noise, inability to localize sound and tremendous effort to communicate throughout the day. This research will meet urgent clinical needs to 1) understand the consequences of unilateral hearing loss, 2) expand criteria for cochlear implantation in one ear, and 3) establish new criteria for bilateral implantation in children and adults. Behavioral and imaging study results will advance understanding of the effects of asymmetric hearing and guide intervention for patients.